FIELD OF THE INVENTION
The present invention relates to a high frequency power amplifier system and a mobile communication system with the high frequency power amplifier system built therein such as a cellular phone, and particularly to a technique effective for application to a high frequency power amplifier system (high frequency power amplifier module) which is used in a phase modulation (PM) system or the like and requires low distortion.
A high frequency power amplifier system has been used in a transmitter of a mobile communication system such as a car phone, a handyphone system or the like.
In digital handyphone systems such as PDC (Personal Digital Cellular), PHS (Personal Handyphone System), N-CDMA (code division multiple access), W-CDMA, etc., a phase modulation system has been adopted as a modulation scheme.
An improvement in distortion characteristic of a power amplifier with respect to a change in temperature is of importance in the phase modulation system. In order to achieve the stabilization of the distortion characteristic, the stabilization (fixing) of a drain current has heretofore been carried out.
As techniques for achieving the stabilization of the drain current, (1) a system for adding parts such as diodes, a thermistor, etc. to a circuit, (2) a system for changing the direction of a crystal axis of an FET (Field-Effect Transistor) to thereby control a change in impedance due to internal stress (piezo) ("Report ED97-182 on Technical Investigations of the Institute of Electronics, Information and Communication Engineers" issued by the Institute of Electronics, Information and Communication Engineers, 1998-01, P9-P14), and (3) a system ("Report MW97-33 on Technical Investigations of the Institute of Electronics, Information and Communication Engineers" issued by the Institute of Electronics, Information and Communication Engineers, 1997-06, P37-P42) for matching an operating point with a bias point (cross point: point Q) at which no drain current changes even if the temperature changes in a voltage (Vg) vs. current (Ids/IdssO) characteristic of each FET, etc. have been adopted.